In order to obtain increase spectral efficiency and therefore increased data throughput for both optical and electrical data transport systems, complex modulation schemes, such as multilevel (ML) signaling, are desired by the communications industry for both electrical and optical domain applications. Clock recovery of the resulting complex waveforms can be difficult, but is critical for data recovery.
Conventional telecommunication clock-recovery (CR) integrated circuits (IC's) are generally only designed for use with binary non-return-to-zero (NRZ) (or equivalently On-Off-Keyed, OOK, waveforms). Conventional CR IC's may employ comparators to determine the clock signal from the multi-level signal. Such an approach usually does not detect a high percentage of the transitions of a multilevel signal that are important to determine the clock signal.
Other conventional clock recovery units directly process multilevel signals to recover the clock signals. However, such units are not useful in the high speed communications environment because of the processing time needed by these conventional units to extract the clock signals from the multi-level signals.
In yet another conventional approach, processing of the multi-level signal is focused on the middle of the multi-level signal and not the edges of the signal. More precisely, the slopes and amplitudes at the temporal decision points of the multilevel signals are examined to align the recovered clock so as to minimize an estimated sampling error. Such an approach focuses on minute signal amplitude variations, where the signal is expected to be flat, rather than major signal transitions, where the signal amplitude should generally experience major changes. The latter property is more indicative of the underlying clock. Also, usually in this conventional approach, the method involves decoding signal values prior to clock recovery. Such decoding is subject to decoding errors.
In view of the foregoing, there is a need in the art for efficiently detecting clock signals in a multilevel signal. There is a further need in the art for a system and method to determine clock signals from a multilevel signal in high speed communication applications.